The present invention relates in general to semiconductor technology, and in particular to power semiconductor devices and methods of manufacturing the same.
Several types of power semiconductor devices, such as transistors and diodes, comprise vertical devices. A vertical device generally comprises a mesa structure formed in a semiconductor epitaxial layer of a semiconductor substrate, where the mesa is defined by one or more trenches disposed in the epitaxial layer and around the mesa. A controllable current flows from the mesa's top to the mesa's bottom, in a direction that is generally perpendicular (e.g., vertical) to the surfaces of the epitaxial layer and semiconductor substrate. The vertical device typically has an ON-state where the current flows through the mesa with a low amount of electrical resistance, and an OFF-state where the current flow is substantially suppressed, except for a very small leakage current. In the OFF-state, the voltage that can be sustained through the mesa while suppressing current flow can be very high, such as in the range of several tens of volts. However, due to various processes occurring in the semiconductor material, such as carrier avalanche, the ability to suppress the current flow in the OFF-state fails at a certain level of voltage, which is often called the breakdown voltage.
The power semiconductor art generally views this breakdown as a detrimental effect that should be avoided during the operation of the device. Great effort is spent on designing power semiconductor devices to increase their breakdown voltages while maintaining low levels of ON-state resistance. The circuit design art generally teaches circuit designers to avoid exposing semiconductor devices to breakdown conditions, and to select devices that have breakdown voltages substantially greater than the voltage levels that the devices will encounter during circuit operation. Exposing a conventional power semiconductor device to repeated breakdown conditions often leads to changes in its electrical characteristics, and sometimes to the destruction of the device.